When producing thick-gauge steel plate by hot rolling, to obtain steel plate superior in mechanical properties and having uniform quality characteristics and shape characteristics, the usual practice has been to convey the finished thick-gauge steel plate while being constrained by constraining rolls and to spray the top surface side and bottom surface side with cooling water to cool the two surfaces of the thick-gauge steel plate so as to stably secure symmetry of temperature distribution in the plate width direction of the thick-gauge steel plate and symmetry of temperature distribution in the plate thickness direction.
Regarding this type of cooling, for example, as shown FIG. 9, arranging a line of nozzles 11s provided with nozzles 11 long in the steel plate width direction at the top surface side of the steel plate 6 conveyed constrained between the constraining rolls 51, 52, each comprised of a top roll 5a and a bottom roll 5b, arranging a line of nozzles 12s provided with more nozzles 12 than the line of nozzles 11s of the top surface side at the bottom surface side, and spraying cooling water from the line of nozzles 11s and line of nozzles 12s on the two surfaces of the steel plate 6 so as to cool the steel plate 6 is disclosed in Japanese Patent Publication (A) No. 11-347629.
In the cooling disclosed in Japanese Patent Publication (A) No. 11-347629, by setting the top surface side line of nozzles 11s and the bottom surface side line of nozzles 12s so as to make the positions in the length direction of the steel plate where cooling water w starts to strike the steel plate 6 between the constraining rolls 51, 52 match at the top surface side and bottom surface side of the steel plate 6, in the cooling process of the steel plate 6, the steel plate 6 is cooled so that the changes in temperature at the fine parts at the top and bottom surfaces become the same (symmetric) about the center plane of thickness of the steel plate 6 as a plane of symmetry.
The top surface side line of nozzles 11s used in the cooling disclosed in Japanese Patent Publication (A) No. 11-347629 is comprised of one line of slit nozzles long in the steel plate width direction. Further, the bottom surface side line of nozzles 12s is comprised of either slit nozzles, spray nozzles, tubular laminar nozzles, tubular spray nozzles with guide pipes, or multihole nozzles.
In the cooling disclosed in Japanese Patent Publication (A) No. 11-347629, as shown in the examples, one line of slit nozzles is arranged at the top surface side, a plurality of lines of slit nozzles, tubular spray nozzles with guide pipes, tubular laminar nozzles, etc. are arranged over a broad region at the bottom surface side, and the entire region of the bottom surface side of the steel plate is sprayed uniformly with cooling water w without regard as to the position with respect to the top surface side line of nozzles and the regions with plate top water present.
Here, in the cooling process of steel plate, the changes in temperatures at the top and bottom surfaces of the steel plate along with time have to be made the same (symmetrical) about the center plane of thickness of the steel plate as a symmetrical plane, but at the top surface side of the steel plate, there are parts which the water sprays from the nozzles strike and parts where plate top water flows. The cooling abilities at the different parts differ, so it is difficult to make adjustments for changes in said temperatures along with time.
The cooling ability is large and stable at the parts which the water sprays strike, but is small at the parts where plate top water flows. This is because the cooling ability with respect to steel plate differs between the case where the water sprays strike from the vertical direction and the case where water flows in parallel along the steel plate.
At the bottom surface side of the steel plate, there are no factors of instability such as plate top water, so cooling is performed uniformly, but at the top surface side of the steel plate, there is a distribution of magnitude of the cooling ability, so balanced cooling from the top surface side and bottom surface side of the steel plate is difficult.
For this reason, symmetry of temperature of the top surface side and bottom surface side of the steel plate sometimes cannot be sufficiently secured. As a result, there is the problem that that uniformity of flatness and quality of the steel plate is difficult to stably secure.
A cooling method aimed at solving the above problem is disclosed in Japanese Patent Publication (A) No. 2004-1082. In the cooling method disclosed in this publication, as shown in FIG. 10, when using constraining rolls 51, 52 to grip and convey high temperature state thick-gauge steel plate and at that time spraying water on the top and bottom surfaces of the thick-gauge steel plate, water is sprayed from one or more lines of top surface side spray nozzles (here, 131 to 136) and lines of bottom surface side spray nozzles (here, 141 to 146) arranged positioned so as to face the top surface side and bottom surface side.
In the case of the cooling method disclosed in Japanese Patent Publication (A) No. 2004-1082, by spraying water so that the total area of the water spray impact parts formed by the lines of bottom surface side spray nozzles on the surface of the thick-gauge steel plate becomes 60% or more of the area of the steel plate in the region between the constraining rolls 51, 52 (substantially region of distance L between centers) and cooling the top and bottom surfaces of the thick-gauge steel plate 6 efficiently and with a good balance, symmetry of the temperatures of the top surface side and bottom surface side of the thick-gauge steel plate 6 is secured, the flatness of the thick-gauge steel plate 6 is improved, and the quality is made uniform.
However, since the area of the water spray impact parts from the lines of spray nozzles arranged positioned facing the top surface side and bottom surface side is made 60% or more of the area of the thick-gauge steel plate area between the constraining rolls 51, 52, in particular, at the top surface side, the case where the area of the large thick-gauge steel plate between the constraining rolls 51, 52 is substantially covered by the water spray impact surfaces is included. A flow resulting from the discharge of the impacting cooling water and interfering convection parts where the sprays interfere and convect are formed unevenly in the width direction of the thick-gauge steel plate. As a result, there is a concern that the cooling efficiency will drop and the cooling will become uneven.
Further, as shown in the cooling method disclosed in Japanese Patent Publication (A) No. 2004-1082, to secure an area of water spray impact parts of 60% or more of the area of the thick-gauge steel plate between the constraining rolls, for example, as shown in FIG. 11, it is necessary to completely cover the horizontal line part by impacting sprays of water and to ensure that the water sprays impact even the hatched regions between the constraining rolls 5 and the thick-gauge steel plate 6.
For this reason, it is necessary to spray water at a slant at the spaces sandwiched between the constraining rolls 5 and the thick-gauge steel plate 6. An apparatus of a complicated structure configured so as to be able to spray water from a large number of spray nozzles becomes necessary. In the final analysis, there is also the problem that the costs of fabrication of the equipment swells.